The present invention relates to a head positioning system having position error compensation and, more particularly, to compensation for erroneous detection of a position error detector in positioning of a disk apparatus such as a floppy disk drive (FDD).
It is well known that, in an FDD which uses a floppy disk as a recording medium and a magnetic head as a head, a step motor is used as an actuator for positioning the magnetic head because a servo mechanism can be simplified and in terms of advantages in the size of an apparatus and the cost.
Recently, as such an FDD apparatus is made compact and its capacity is increased, a pitch of data tracks (tracks per inch) of a magnetic disk is reduced, and therefore eccentricity of the data track cannot be ignored. For this reason, positioning accuracy is improved by controlling the step motor not by an open loop but in a feedback manner using a sector servo system.
In the sector servo system, as is well known, the data surface of a magnetic disk is divided into several sectors, and track position error information is written in a start position or an end position of the sector, thereby obtaining track position error information at each sampling time determined by the number of sectors and the rotational frequency of the disk. Then, the magnetic head is feedback-controlled on the basis of the information obtained at the sampling point. U.S. patent application No. 921514 (now U.S. Pat. No. 4,788,608) filed on Oct. 22, 1986 (European Patent Application No. 86114617.3) discloses a method of performing the sector servo system. That is, in this method, a digital filter for generating a sine wave having the same frequency as that of a second frequency component in a variation component in a target data track with respect to an impulse input is provided in a feedback loop, thereby positioning the magnetic head at a position of the target data track with high accuracy.
However, in this method, tracking control of the magnetic head with respect to a data track is performed on the basis of position error information obtained at only each sampling time determined in accordance with the number of sectors and the rotational frequency of the disk. Therefore, if the position error signal itself does not represent a correct value due to noise or defects in the medium, the tracking characteristic of the magnetic head with respect to a data track is degraded when the magnetic head is positioned in accordance with the position error signal.